UC Santa Barbara

Sea urchins are voracious omnivorous grazers that are capable of dramatically altering coastal marine ecosystems worldwide, including kelp forests. Changes in urchin grazing patterns due to their abundances, behavior, or food availability can result in transitions between lush, biodiverse kelp forests and urchin barrens denuded of macroalgae. In the Santa Barbara Channel, two species of sea urchins predominate: the red urchin Mesocentrotus franciscanus and the purple urchin Strongylocentrotus purpuratus. To date, the majority of ecological research in this region has focused on a single urchin species or aggregated both. However, these two species differ in many ways that are highly relevant to their ecological role, including in their morphology, distribution, competitive ability, and vulnerability to predation and harvest. I used an extensive long-term population dataset to assess how these two major grazers are differentially influenced by management (chapter 1) and the environment (chapter 2). I also combined laboratory studies of foraging rates on common understory algae species with long-term data on kelp forest community structure to evaluate the ecological implications of these differences (chapter 3). Lastly, I contextualized the results of my feeding study by quantifying the interspecific differences in metabolism between red and purple urchins fed different algal diets (chapter 4).

My results demonstrate that species identity cannot be discounted when assessing the management and impact of these key ecosystem engineers. While red urchin populations have increased inside some marine reserves where they are protected from fishing pressure, purple urchin populations displayed no reserve effect; rather, they were associated with heat wave events and declines in kelp density. Purple urchins also had significantly higher foraging rates than red urchins proportional to their size, indicating the potential ecological impact of these populations on the understory algae community could exceed that of red urchin dominated systems. This work lays the foundation for more accurately parameterized models of urchin populations, allowing us to make better predictions of urchin grazing and the factors affecting urchin barren formation and recovery.